An electret microphone is widely applied to devices such as a mobile phone and a recording pen due to the advantages of small volume, simple structure, wide frequency response, high sensitivity, vibration resistance and low price. Particularly, the electret microphone is popularized in a high-end recording device. An operating principle of the electret microphone is as follows: when an electret diaphragm vibrates due to an acoustic wave, a change in a distance between the electret diaphragm and a metal polar plate may be caused. That is, a capacitance of a capacitor formed between the electret vibrating diaphragm and the metal polar plate changes with the acoustic wave, causing a change in an inherent electric field across the capacitor, thereby generating an alternating voltage changing with the change in the acoustic wave. Since the “capacitor” formed between the electret diaphragm and the metal polar plate has relatively small capacitance, the output impedance thereof is high, which is about tens of megohms or more. Such high impedance fails to directly match an input end of a general audio amplifier, so that a field effect transistor is provided in the MIC for performing impedance conversion. An internal circuit of the electret microphone is shown in FIG. 1. A voltage across the “capacitor” is extracted by the field effect transistor with very high input impedance, and is amplified, so that an output voltage signal corresponding to the acoustic wave is obtained.
The performance of the electret MIC is largely dependent on an operating state of the field effect transistor built in. Therefore, the state of the field effect transistor in the circuit not only determines whether the microphone can work normally, but also determines the operating performance of the microphone. However, the state of the field effect transistor in the circuit depends on amplitudes of a bias resistor RL and a supply voltage U. The bias resistor and the supply voltage are arranged to provide a direct current bias to the field effect transistor amplifier, so as to ensure that the field effect transistor operates in a constant current region (a linear active region). If an operating bias voltage UDs of the MIC is too high or too low, cut-off distortion or saturation distortion will occur with respect to a large input signal. However, characteristics of the field effect transistor in the MICs of different batches and different manufacturers are different, so that the operating bias voltages UDs of the MICs are different as well, while the operating bias voltages of the MICs will affect the quality of voice signals output by the MICs. Therefore, it is an urgent problem to be solved to keep the bias voltage UDs of the field effect transistor built in, or disposed in, the electret microphone within a certain voltage range.